1. Field of the Invention
The present invention relates generally to electronic devices for protecting against short-circuit conditions in circuits comprising semiconductor power elements and, more particularly, to protection devices for push-pull output stages designed to protect against short-circuit conditions between an output terminal and a positive pole of the supply.
2. Discussion of the Related Art
Push-pull output stages can, for example, be found in a variety of applications, such as in audio power amplifiers. In general, the requirements for protection devices, when integrated with the circuit to be protected, are that they should be simple and economical, that they should not lead to losses of working power and that they should not limit in any way the dynamic operation of the circuit itself. The circuit diagram of a known protection device, normally used in monolithically integrated amplifier circuits, for protection against a short-circuit condition between the output terminal of the push-pull output stage and the positive pole of the supply, is shown in FIG. 1 of the drawings.
The push-pull output state of FIG. 1 comprises two bipolar power transistors, T.sub.1 and T.sub.2, both of the n-p-n type, the first transistor T.sub.1 having its collector terminal coupled to the positive pole +V.sub.cc of a supply voltage generator and the second transistor T.sub.2 having its emitter coupled to the negative pole -V.sub.cc of the generator. The point A of the coupling between the emitter of transistor T.sub.1 and the collector of transistor T.sub.2 forms an output terminal of the output stage to which there is coupled, via a capacitive element, a first terminal of a load impedance R.sub.L, the second terminal of which is coupled to the negative pole -V.sub.cc.
The power transistors T.sub.1 and T.sub.2 are driven to conduct in phase opposition by means of circuit control means (not shown) which are coupled to their base terminals. The protection device comprises a bipolar transistor T.sub.4 of the p-n-p type, whose emitter terminal is coupled to the output terminal A and a bipolar transistor T.sub.6 of the n-p-n type, whose base terminal is coupled to the collector of the transistor T.sub.4 and, via a resistor R.sub.6, to the negative pole -V.sub.cc. The collector terminal and the emitter terminal of the transistor T.sub.6 are respectively coupled to the base terminal of the power transistor T.sub.2 and to the negative pole -V.sub.cc. The base terminal of the transistor T.sub.4 is coupled to the positive pole +V.sub.cc, via two series-connected diodes D.sub.1 and D.sub.2 and to the negative pole via a constant current generator G.sub.1. The base terminal of the transistor T.sub.4 is therefore coupled to a voltage reference which sets a threshold for the conduction of the transistor. The transistor T.sub.4 can only begin to conduct when the output terminal A is brought to a potential lower than that of the potential of the positive pole +V.sub.cc by a value almost equal to the base-emitter voltage normally applied to a bipolar transistor in conduction.
Because the power transistor T.sub.1 is never caused to conduct in saturation, the transistor T.sub.4 can only conduct when there is a short-circuit between the output terminal A and the positive pole +V.sub.cc or an output overvoltage. The conduction of the transistor T.sub.4 then also causes the conduction of the transistor T.sub.6, which cuts off the power transistor T.sub.2 thereby protecting the push-pull amplifier output stage.
In fact, if the transistor T.sub.2 remained on during the short-circuit, it would be damaged, possibly irrepairably, as it would simultaneously be subject to a collector-emitter voltage equal to the supply voltage and to a current flux equal to the maximum short-circuit current.
Naturally, if the short-circuit conditions are discontinued the transistors T.sub.4 and T.sub.6 are cut off and the output stage automatically resumes normal operation.
A protection device of the type described above operates reliably, with no drawbacks, when the load impedance is of a resistive or capacitative type, but is not suitable for an ohmic-inductive type load constituted, for example, by a loudspeaker controlled by the output stage of an audio amplifier.
In fact, when an inductive load impedance is coupled to the output terminal of a push-pull output stage, the abrupt variations at certain frequencies of the current which flows in this load, induce therein a counter-electromotive force which may raise the value of the potential of the output terminal beyond the value of the potential of the positive pole of the supply.
The protection device described above then comes into operation, although there is no short-circuit, and cuts off the lower power transistor of the output stage while it is conducting normally, thereby increasing the output overvoltage, with a consequent considerable distortion of the wave shape of the output signal.
A need has therefore been felt for a protection circuit for push-pull output stages that can function properly fon an inductive-type load impedance while protecting the push-pull power output stages against short-circuit conditions.